(a) Field of the Invention
The present invention relates to an antenna for a radio frequency identification (RFID) tag.
The present invention was supported by the IT R&D program of MIC/IITA [2006-S-023-02, Development of Advanced RFID System Technology].
(b) Description of the Related Art
A radio frequency identification (RFID) tag is used in various fields such as distribution and material handling industries, together with an RFID reader.
When an object to which the RFID tag is attached accesses a read zone of the RFID reader, the RFID reader transmits an interrogation signal to the RFID tag by modulating an RF signal having a specific frequency, and the RFID tag responds to the interrogation of the RFID reader.
That is, the RFID reader transmits an interrogation signal to the RFID tag by modulating a continuous electromagnetic wave having a specific frequency, and the RFID tag transmits back the electromagnetic wave transmitted from the RFID reader after performing back-scattering modulation in order to transmit its own information stored in the RFID tag's internal memory. The back-scattering modulation is a method for transmitting tag information by modulating the amplitude and/or the phase of a scattered electromagnetic wave when the RFID tag transmits the electromagnetic wave that is initially transmitted from the RFID reader back to the RFID reader by scattering the electromagnetic wave.
Since a passive RFID tag does not include a separate operation power source, it rectifies the electromagnetic wave transmitted from the RFID reader and uses the rectified electromagnetic wave as its own power source to acquire operation power. The intensity of the electromagnetic wave transmitted from the RFID reader should be larger than a specific threshold value for normal operation. However, since the transmission power of the reader is limited by local regulations of each country, it is not possible to unconditionally raise the level of transmission power.
Therefore, the RFID tag should efficiently receive the electromagnetic wave transmitted from the RFID reader to extend the read zone without raising the transmission power level of the reader. A method for raising the receiving efficiency of the RFID tag is to perform complex conjugate matching of an antenna and a radio frequency (RF) front-end of the RFID tag chip so as to maximize the intensity of the signal received by the RFID tag.
A conventional RFID tag will be described in detail with reference to FIG. 1.
FIG. 1 is a configuration of a conventional RFID tag.
As shown in FIG. 1, the RFID tag includes an RFID tag chip 10 and an RFID tag antenna 20 (hereinafter referred to as “an RFID tag antenna”) for an RFID tag.
The RFID tag chip 10 stores information on an object to which the RFID tag is attached, and modulates the amplitude and/or the phase of an electromagnetic wave transmitted from an RFID reader for transmitting the information of the object. The RFID tag chip 10 modulates the amplitude and/or the phase of the wave by controlling the amount of power through input impedance, and includes an RF front-end that has input impedance.
The RFID tag antenna 20 scatters the electromagnetic wave that is modulated by the RFID tag chip 10. The RFID tag antenna 20 includes a dielectric material 21, a feed loop 23, radiating patches 25, and shorting plates 27.
The dielectric material 21 is rectangular-shaped with a relatively low dielectric constant, and a bottom surface of the dielectric material 21 is a ground surface that contacts the object.
The feed loop 23 is formed in an upper surface of the dielectric material 21, and is electrically connected to the RFID tag chip 10 so as to supply power thereto.
Each of the radiating patches 25 is formed in the upper surface of the dielectric material 21, and excites a current having an out-of-phase characteristic by using a current flowing through the feed loop 23 and radiates the excited current.
Each of the shorting plates 27 is formed on a part of a side of the dielectric material 21 and connects the radiating patches and the ground surface. That is, the shorting plates disconnect the radiating patches 25 and the ground surface.
Generally, in an RFID system including an RFID tag and an RFID reader, transmission power of the RFID reader is limited by local regulations of each country. Therefore, in order to extend a read zone of the RFID reader, the RFID tag antenna should have high efficiency, the RFID tag should resonate at a corresponding frequency, and the RFID tag antenna and the RF front-end of the RFID tag chip should be complex-conjugate matched.
However, the conventional RFID tag shown in FIG. 1 is not provided with a method for controlling impedance-matching of RFID tag chips that have various impedance characteristics.
Further, there are difficulties in miniaturizing the RFID tag antenna and reducing cost.